This invention relates to a variable camshaft timing (VCT) system for an automotive engine in which the circumferential position of an engine camshaft is varied relative to the crankshaft, or to another camshaft of the engine, by controllably transferring hydraulic fluid between opposed operators that cooperatively act to reposition the camshaft. More particularly, this invention relates to a VCT system of the foregoing character in which the opposed hydraulic operators are in the form of a diametrically opposed spaced apart pair of lobes of a vane that is secured to the camshaft whose position is to be varied.
U.S. Pat. No. 5,002,023 (Butterfield and Smith) and U.S. Pat. No. 5,046,460 (Butterfield, Smith and Dembosky) describe various forms of VCT systems that utilize opposed hydraulic actuators in the form of opposed cylinders to circumferentially reposition an automotive engine camshaft relative to a crankshaft, or relative to another camshaft of the same engine, by controllably transferring pressurized engine lubricating oil from one of the hydraulic cylinders to the other. Further, as is disclosed in U.S. Pat. No. 5,107,804 (Becker, Butterfield, Dembosky, and Smith), the disclosure of which is incorporated by reference herein, a VCT system using the principles of the aforesaid U.S. patents can be simplified in its mechanical and hydraulic aspects by using a vane with a diametrically spaced apart pair of lobes secured to the camshaft in combination with a surrounding housing that is oscillatable with respect to the camshaft, in place of the opposed hydraulic cylinders of the aforesaid patents. An arrangement in which the phase angle of an engine camshaft relative to that of the crankshaft is adjusted by hydraulic action against a series of vanes that are secured to the camshaft and are free to oscillate within chambers of a drive member that is driven in rotation by the crankshaft is also disclosed in U.S. Pat. No. 4,858,572 (Shirai et al.).
A vane-type hydraulically operated VCT system of the type described in the aforesaid U.S. Pat. No. 5,107,804 relies on the presence of pressurized engine lubricating oil or other hydraulic fluid within the VCT system to function properly and predictably. This condition is achieved during normal engine operation, when engine lubricating oil is used as the hydraulic fluid within the VCT system, since adequate pressurization of the engine lubricating oil inherently results from the operation of the engine. However, when an automotive engine is shut off, the pressure of the engine lubricating oil soon drops, and the oil within a VCT system of the aforesaid type will normally drain back to the engine crankcase. Thus, upon the restarting of such an engine, the supply of engine lubricating oil within the VCT system is likely to be inadequate in volume or pressure to ensure its proper operation. During these conditions it is desirable to be able to automatically lock or latch the position of the phase adjusted camshaft relative to that of the crankshaft in a predetermined position, preferably in a centered position between its fully advanced position and its fully retarded position, and to maintain the phase adjusted camshaft in such a locked or latched condition unless the pressure of the engine lubricating oil within the VCT system is adequately high to ensure normal, proper operation of the VCT system.
According to the present invention there is provided an hydraulically-operated, vane-type variable camshaft timing (VCT) system for an automotive engine in which the vane that is secured to the camshaft is positively locked or latched in its position with respect to a crankshaft driven, rotatable housing during periods of low hydraulic system pressure, for example, during and shortly after engine starting in the case of a VCT system that uses pressurized engine lubricating oil as the hydraulic medium. The housing carries a pin therein that is slidable to and fro along an axis that is spaced from and extends parallel to the axis of rotation of the phase adjusted camshaft. Further, the camshaft has a plate that is secured thereto and that is rotatable therewith, and the plate has a recess therein that is adapted to receive a free, rounded end of the pin of the housing when the pin is circumferentially aligned with the recess and when the pin is at the outer limit of its range of travel. The pin is resiliently biased to the outer limit of its travel by a spring that acts on an opposed end of the pin, and is further biased away from the outer end of its range of travel, and thereby out of engagement with the recess, by pressurized hydraulic fluid within the recess that acts on the free end of the pin during the operation of the engine. The relationship between the hydraulic force that acts on the free end of the pin and the mechanical, spring force that acts on its opposed end is such that the hydraulic force is substantially greater during all normal operating conditions of the engine and the mechanical force is greater only temporarily upon restarting of the engine. Thus, in a VCT system according to the present invention the camshaft is mechanically locked or latched in a predetermined phase relative to the crankshaft, preferably in a centered phase between its fully advanced position and its fully retarded position, to ensure proper starting of the engine at a time when the VCT system could otherwise function unpredictably and unreliably because of inadequate hydraulic pressure within the system. Further, the VCT system according to the present invention prevents impacts between the VCT system components during and shortly after starting, when the system can be fully or partly filled with air and when such impacts could occur because of the compressibility of air and the sensitivity of a VCT system of the type described in the aforesaid U.S. Pat. No. 5,107,804 to torque fluctuations during each rotation of the camshaft. Thus, the latching or locking feature of the VCT system of the present invention prevents such impacts, and the damage and noise resulting therefrom, by ensuring that no phase adjustment can occur until the VCT system is filled with pressurized fluid and the associated check valves can function properly to prevent hydraulic fluid transfer within the system except when desired to effect a change in camshaft phase angle.
Accordingly, it is an object of the present invention to provide an improved vane-type hydraulic variable camshaft timing (VCT) system. More particularly, it is an object of the present invention to provide a VCT system of the foregoing character with a mechanical locking feature that will function to prevent changes in camshaft phase angle whenever the pressure of the hydraulic fluid within the VCT system is inadequate to ensure its proper operation. Specifically it is an object of the present invention to provide a VCT system of the foregoing character that utilizes pressurized engine lubricating oil as the hydraulic medium, and incorporates a locking or latching feature to prevent changes in camshaft phase angle during and shortly after the restarting of the engine, when the pressure of the engine lubricating oil within the variable camshaft timing system is inadequate to ensure its proper operation.